In the modern society, especially in the societies of advanced countries, opportunities of ingesting high calorie and high fat diet are increasing. For this reason, mortalities due to ischemic diseases resulting from arteriosclerosis (heart diseases such as myocardial infarction and angina pectoris, cerebrovascular diseases such as cerebral infarction and cerebral hemorrhage) have been increasing. Therefore, it has been desired to diagnose such conditions at an early stage to employ an appropriate treatment. However, no satisfactory method is available for diagnosing progress of arteriosclerosis at an early stage before the onsets of the aforementioned diseases.
Methods for diagnosing arteriosclerosis are basically classified into non-invasive methods and invasive methods in which a catheter or the like is inserted into an artery. Among them, typical non-invasive methods include X-ray angiography and ultrasonography. However, by these methods, it is almost impossible to detect arteriosclerosis at an early stage, especially constriction of coronary artery, which causes myocardial infarction or angina pectoris, at an early stage before the onset of these diseases.
CT, MRI and the like may sometimes be used as another class of non-invasive methods. However, these methods have been mainly developed for detection of tumors, and accordingly, they have a problem of a low resolution of arteriosclerotic lesions. In addition, the methods require expensive and large-scale apparatuses, which limits employable hospitals and general applicability. Further, methods utilizing radioisotopes have also been investigated. However, these methods still remain at an experimental level.
As the invasive methods, intravascular echo, vascular endoscope and the like have been used. It is recognized that an arteriosclerotic lesion with a thickness as thin as 0.1 mm can be measured by these methods. However, for employment of these methods, it is necessary to arterially insert an ultrasonic oscillator or an endoscope attached to an end of a catheter, which may result in serious physical stress and heaviness as well as a risk of a patient. Therefore, although these methods have been used therapeutically for patients after the attack of myocardial infarction and the like or as secondary prophylaxis, they cannot be used for a diagnostic purpose to know as to presence or absence or a degree of progress of arteriosclerosis in a patient before onset.
Among the aforementioned methods, a method most widely used for identification of a lesion of arterial vasoconstriction is X-ray angiography. This method comprises the step of administration of a water-soluble iodine contrast medium to visualize vascular flows, and detecting a lesion at which the flows are obstructed. However, these methods can only detect a lesion where constriction progresses 50% or more and fail to detect a lesion before the onset of attack of an ischemic disease.
Separately from the above methods, attempts have also been reported in which a hydrophobic iodine contrast medium or a hydrophilic contrast medium is formulated for selective accumulation in a target lesion (International Patent Publications WO95/19186, WO95/21631, WO89/00812, British Patent No. 867650, WO96/00089, WO94/19025, WO96/40615, WO95/2295, WO98/41239, WO98/23297, WO99/02193, WO97/06132, U.S. Pat. Nos. 4,192,859, 4,567,034, 4,925,649, Pharm. Res., 16 (3), 420 (1999), J. Pharm. Sci., 72 (8), 898 (1983), Invest. Radiol., 18 (3), 275 (1983). For example, Pharm. Res., 16 (3), 420 (1999) discloses that, by injection of an oil-particle dispersion of cholesteryl iopanoate as a hydrophobic compound, the iodine compound accumulates in arteriosclerotic lesions of experimental animals.
Further, J. Pharm. Sci. 72 (8), 898 (1983) discloses examples of X-ray hepatography and splenography by injection of an oil-particle dispersion of cholesteryl iopanoate. U.S. Pat. No. 4,567,034 describes a method of selective hepatography or splenography utilizing liposomes encapsulating an ester of diatrizoic acid. International Patent Publications WO96/28414 and WO96/00089 disclose contrast media for imaging vascular pools or lymphatic systems. However, the methods using these formulations are not satisfactory in efficiency and selectivity for a purpose of selective contrast of vascular diseases, and no example thereof is reported in which vascular diseases are imaged by utilizing X-ray irradiation.
Mechanisms of onset of arterial diseases have recently been progressively elucidated at levels of genes, proteins and cells (J. Biol. Chem., 1996, 271 (44) 27346–52; Nature, 386 (6662) 292–6). As for arteriosclerosis, it has been elucidated that plural kinds of cells form lesions while they mutually control their proliferation (Arterioscler. Throm. Vasc. Biol., 1999 (3) 461–71; Lab Invest 1998, 78 (4) 423–34). However, no example has been demonstrated which reproduces the state of a lesion, in which plural kinds of cells are involved as mentioned above, in a cell culture vessel, and accordingly, evaluation of drugs for arteriosclerosis or restenosis have so far been conducted mainly by using model animals.
However, such methods using animals are time-consuming and require a high cost, and their use is also required to be minimum from a viewpoint of prevention of cruelty to animals. Therefore, an in vitro evaluation method of reproducing a state of arteriosclerotic lesion has been desired for screening of a large number of compounds for a short period of time.